The present invention relates to a process for preparing suede sheet materials which feel soft and are attractively colored. More particularly, the invention relates to a process for dyeing densely or clearly, without damage to tactile properties, a suede sheet material comprising a fibrous mat and a binder composed mainly of an elastomer, on the surface of which a nap of fibers is formed by buffing, brushing or carding.
Known suede sheet materials are generally divided into two types. One type is formed by buffing the surface of a porous film composed of an elastomer to expose a porous polymeric layer. Suede sheet materials of this type are disclosed in, for example, U.S. Pat. No. 3,483,015, U.S. Pat. No. 3,616,023, U.S. Pat. No. 3,567,535, U.S. Pat. No. 3,284,274 and U.S. Pat. No. 3,429,727.
Another type is formed by impregnating a fibrous mat with an elastomeric binder, coagulating the elastomer to form an impregnated mat and buffing the impregnated mat to form a nap of fibers at the surface. Suede sheet materials of this latter type are disclosed in, for example, U.S. Pat. No. 3,067,482 and British Pat. No. 914,712.
In suede sheet materials of the former type, the nap is solely an elastomeric material which does not have surface scratch resistance sufficient to permit the sheet material to be effectively used as a leather substitute. In addition, suede sheet materials of the former type do not feel like genuine suede material, cannot be deeply colored and do not show a clear writing effect. The writing effect is the visual effect which occurs when the lay of the nap is changed. Surface characteristics such as color hue and luster are changed according to the viewing angle.
In suede sheet materials of the latter type, the fibrous nap exhibits a very high surface scratch resistance. Further, the use of finer fibers in the nap brings about advantages. For example, the surface feels more like genuine suede. Also where a suede sheet material is formed by using mixed-spun fibers and one component of the mixed-spun fibers is extracted, the nap of the remaining ultra-fine fibers has excellent tactile properties and shows a clear writing effect.
However, suede sheet materials of the latter type have a serious drawback. Since the sheet material is a composite material of two materials having different properties; namely, a fibrous material which constitutes the nap and an elastomeric material which constitutes the binder for the sheet material, applying surface coloring by dyeing or the like produces an unevenly colored finish due to differences between the two materials in chemical properties such as dyeability, color development and color fastness.
Further, if the fibers in the nap become too fine (for example, 0.1 denier or finer) good color development cannot be obtained in the fiber component when the suede sheet material is dyed. Therefore, the suede sheet material cannot be colored densely and clearly.
If the size of the fibers is further reduced below 0.07 denier, extreme color unevenness or scattering is caused and island-like patterns occur across the surface. The commercial value of such unevenly colored material is drastically lowered.
Poor color development of the fibers may be improved in some cases by dyeing the fibers more densely. However, if only the fibers are dyed densely; the color has a tendency to fade when subjected to rubbing, washing or light. Moreover the surface becomes hard and a good writing effect and a desirable tactile cannot be obtained.
To at least partially eliminate such defects in suede sheet materials of the latter type, a pigment may be incorporated in advance into the elastomer to be impregnated in the fibrous mat. However, when this method is applied on an industrial scale, it is very difficult to frequently change pigments. Accordingly, although mass production of suede sheet material of the same color is possible, production of small quantities of a variety of products, which is standard practice in industries using this type of material, cannot be accomplished conveniently. This method has the further drawback that color matching is very difficult even if a post dyeing technique is adopted. Moreover, when the thickness of the nap-constituting fibers is reduced, the fibers do not accept dye as well as the elastomer and appear rather whitish against the background of the elastomer.
Another method for overcoming some of the foregoing defects is to use two different dyes having good color development properties for the nap and the binder component, respectively. The dyeing operation is conducted twice. With this method, however, color matching is very difficult, although a product having a good color development can be obtained. Further, this method provides no substantial solution to the above-mentioned problems caused by reduction of the fiber fineness.
Where a suede sheet material has ultrafine (e.g. smaller than 0.07 denier) fibers of various thickness, the above-mentioned dyeing method cannot prevent the occurance of uneven dying amoung the fibers. Color scattering is inevitably brought about, resulting in reduction of the commercial value of the material. The undesired phenomenon of color scattering is a serious defect of suede sheet materials of the latter type and cannot be overcome by any of the conventional methods.